1. Field of the Invention
The invention relates in general to electrical inductive apparatus, such as transformers, and more particularly to the removal of residual polychlorinated biphenyl from the internal components in electrical inductive apparatus.
2. Description of the Prior Art
Since the early 1930's, electrical transformers used in locations sensitive to fires or fire-damage such as subways, buildings and factories have been constructed with a polychlorinated biphenyl insulating and cooling liquid, which liquids are commonly called PCB's. The PCB's were chosen for these applications because of their high dielectric strength and their fire resistant characteristics.
In 1976, the manufacture of PCB was outlawed in the United States (15 U.S.C.A .sctn.2605 (3) (A)(i)) because of evidence of their carcinogenic nature. The Federal Toxic Substances Control Act has made it mandatory that the use of PCB's in industry be phased out over a short period of time. The Environmental Protection Agency has determined that PCB concentrations of 50 ppm or less in the dielectric fluid of a transformer are considered safe for transformer operation. The EPA has further designated that a PCB transformer may be re-classified as "Non-PCB" if after decontamination is completed (and disengaged) for 90 days, the residual PCB concentration in the dielectric fluid is below 50 ppm.
Because initial PCB concentrations in these transformers was as high as 600,000-1,000,000 ppm and the PCB's impregnate the solid cellulosic insulation (wood and paper) and other adsorbent insulating materials used in transformers, merely flushing the transformer with another dielectric fluid or a solvent may have the affect of immediately reducing the PCB concentration to an acceptable level, but after a period of operation, the concentration will rise above the limit set by the EPA due to the concentrated PCB's continuously leaching out of the solid insulation.
The prior art purports to teach a method of removing PCB's from transformers through the use of an activated carbon filter located in a thermal siphon attached to the transformer while it is energized (U.S. Pat. No. 4,124,834). The activated carbon filters have a finite ability to absorb PCB's. It is therefore necessary to continually change out the activated carbon filters and monitor the concentrations of PCB's. The process is continued until the concentration of PCB in the dielectric fluid is below 50 ppm. Although able to reach 50 ppm in approximately 30-60 days, when disengaged from the transformer, the fluid which is a poor solvent for PCB, rapidly leaches back to concentration well above 50 ppm. To date, this process has been operated continuously on transformers for two (2) to three (3) years without successfully keeping the PCB concentration below 50 ppm after disengagement.
There is also in the prior art a process which appears to suggest circulation of a chlorinated or halogonated aliphatic hydrocarbon vapor through the transformer (U.S. Pat. No. 4,425,949). Equipment required for this method include two pumps, one decanter, one thermosiphoned reboiler, two inert chillers, one condenser, one superheat exchanger, one reservoir and an optional distillation vessel. The requirement of this quantity and complexity of equipment is apparently dictated by the fact that the transformer cleansing is performed in vapor rather than liquid phase. This magnitude of complexity would obviously create high initial costs, high operating costs and high maintenance costs. Also, the process described in U.S. Pat. No. 4,425,949 must be practiced while the transformer is out of service because existing PCB transformer are not designed to operate in a dielectric gas atmosphere and the resulting lack of heat dissipation would cause the tranformer to fault or melt down. The inability to operate the transformer while decontamination is taking place precludes the heating of and subsequent expansion of the transformer windings and core. The non-energized condition excludes the vapor cleansing process of U.S. Pat. No. 4,425,949 from access to internally trapped PCB which will remain there until the transformer is refilled and re-energized.